rtpmanager: rtpsession: don't send empty RTCP packets
rtpmanager: rtpsession: don't send empty RTCP packets

generate_rtcp can produce empty packets when reduced size RTCP is turned on.
Skip them since it doesn't make sense to push them and they cause errors with
elements that expect RTCP packets to contain data (like srtpenc).

pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good